1. Field of the Invention
This invention relates to a piezoelectric ceramic composition that is widely utilized in the field of actuators, sensors, resonators, and the like.
2. Description of the Related Art
The piezoelectric materials (piezoelectric ceramic compositions) possess an effect of generating strain when an electric field is applied thereto from outside (conversion of electric energy to mechanical energy) and an effect of generating an electric charge on the surface thereof when stress is applied thereto from outside (conversion of mechanical energy to electric energy) and have been widely utilized in various fields in recent years. Such a piezoelectric material as lead titanate zirconate (Pb(Zr, Ti)O3: PZT) excels in minute positional justification and finds utilization as in optical minute justification because it generates strain substantially proportional to the order of 1×10−10 m/V in response to an applied voltage. In contrast thereto, the piezoelectric material is utilized also as a sensor for reading out minute force and deformation because it generates a large electric charge in proportion to the stress exerted thereon or the amount of deformation of itself caused by the stress. Further, since the piezoelectric material possesses excellent responsiveness, it is capable of enabling the piezoelectric material itself or an elastic body adapted for union with the piezoelectric material to be excited and consequently allowed to induce resonation therewith and, therefore, is utilized as a piezoelectric transformer, a supersonic motor, etc.
Most piezoelectric materials now available for practical applications are solid solution systems (PZT systems) consisting of PbZrO3(PZ)-PbTiO3(PT). The reason for this fact is that excellent piezoelectric properties can be obtained by using a composition nearing the morphotropic phase boundary (M. P. B.) between the rhombohedral crystal-based PZ and the tetragonal crystal-based PT. The PZT-based piezoelectric materials that are adapted to suit various uses in consequence of the addition of a varying auxiliary component or additive have been developed widely. They are varied to such an extent of embracing use as an actuator for positional justification requiring a large voltage coefficient (d) instead of manifesting a small mechanical factor of merit (Qm) and expected to produce a large displacement in the application using direct current and use as a supersonic wave generating device like a supersonic motor possessing a large mechanical factor of merit (Qm) instead of manifesting a small piezoelectric constant (d) and suiting the way of using an alternating current.
Also other piezoelectric materials than the PZT-based materials have been developed for practical applications. They are mostly solid solutions that have as main components such lead-based perovskite compositions as lead magnate niobate [Pb(Mg, Nb)O3:; PMN).
The piezoelectric materials developed for practical applications are invariably lead-based piezoelectric materials as described above and have lead oxide (PbO) extremely rich in volatility even at low temperatures contained as a main component in a large amount nearing 60 to 70 mass %. PZT or PMN, for example, contains lead oxide in an amount of about ⅔ in mass ratio. The lead-based piezoelectric materials that contain lead in such a large amount entail many problems in terms of environmental resistance like public nuisance and from the ecological point of view. During the manufacture of a lead-based piezoelectric material, for example, an extremely large amount on the industrial level of lead oxide is suffered to volatilize and diffuse into the air in the step of a thermal treatment such as firing when the products are ceramic articles or melting when the products are single crystal articles. Though the lead oxide emitted in the step of manufacture may be recovered, the lead oxide contained in piezoelectric products marketed as commercial articles is difficult of recovery in the present situation. When this lead oxide is widely released in the environment, the elution of lead caused by acid rain arouses anxiety. In consideration of the growth of the amounts of their application, therefore, the liberation of lead from the piezoelectric materials proves to constitute an extremely important problem.
As piezoelectric materials that contain absolutely no lead, barium titanate (BaTiO3) and bismuth-bedded ferroelectric materials have been known. The barium titanate, however, is devoid of serviceability in view of applications that involve joining with solder and mounting on a vehicle because it has such a low Curie point as 120° C. and suffers loss of piezoelectricity at a temperature exceeding it. On the other hand, the bismuth-bedded ferroelectric materials, though usually possessing a Curie point exceeding 400° C. and excelling in thermal stability, entail the problem in terms of productivity because it possesses large crystal anisotropy and requires spontaneous polarization to be oriented as by hot forging. Generally, the elimination of lead in a piezoelectric material leads to degradation of piezoelectric property. When the lead content is thoroughly eliminated from the conventional piezoelectric material, for example, it is judged that large piezoelectricity is obtained with difficulty.
Further, in search of a new piezoelectric material, studies have been being promoted on the sodium bismuth titanate-based materials in recent years. JP-B-4-60073 and JP-A-11-180769, for example, disclose materials that contain sodium bismuth titanate and barium titanate and JP-A-11-171643 discloses materials that contain sodium bismuth titanate and potassium bismuth titanate. Then, JP-A-16-035350 discloses systems that contain sodium bismuth titanate, potassium bismuth titanate, and a third component.
These sodium bismuth titanate-based materials, however, have failed to acquire adequate piezoelectric properties as compared with lead-based piezoelectric materials and consequently are required to attain further improvements in the piezoelectric properties in the factual state of affairs. In this situation, the present patent applicant has proposed a piezoelectric ceramic material that contains a first compound possessing a rhombohedral crystal-based perovskite structure, a second compound possessing a tetragonal crystal-based perovskite structure, and a third compound including bismuth (Bi), a divalent metallic element such as manganese (Mg), a tetravalent metallic element such as zirconium (Zr), and oxygen (O) (refer to JP-A-2005-47745, JP-A-2005-47746, JP-A-2005-47748). He has also proposed a piezoelectric ceramic article that contains a first compound possessing a rhombohedral crystal-based perovskite structure, a second compound possessing a tetragonal crystal-based perovskite structure, and a third compound including bismuth, iron (Fe), a pentavalent metallic element like tantalum (Ta), and oxygen (O) (refer to JP-A2005-47747). The piezoelectric ceramic articles disclosed in Patent JP-A-2005-47745, JP-A-2005-47746, JP-A-2005-47748, and JP-A2005-47747 are capable of amply improving such piezoelectric properties as displacement and copiously enhancing the applicability of a non-lead-based piezoelectric material.
Incidentally, the piezoelectric material is required to possess a large piezoelectric constant (d) and a large displacement and meantime required to allow firing to proceed at a low temperature. When a piezoelectric device is formed with a piezoelectric ceramic composition, the formation of an electrode is necessary. The firing at a low temperature, however, is indispensable when this electrode is formed with such an inexpensive electrode material as Ag in the place of an expensive noble metal. From this point of view, the non-lead-based piezoelectric materials disclosed in the aforementioned patent documents are inadequate and hardly realize excellent piezoelectric properties when the firing is carried out at a low temperature.